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My father told me that in 1938, talking with his grandfather and found that age was expressed for each of the last two digits of years that had born. So, when my father was born, the age of your grandfather was:

My father told me that in 1938, talking with his grandfather and found that age was expressed for each of the last two digits of years that had born. So, when my father was born, the age of your grandfather was...

Yes, that is rather confusing. If you are translating from another language (Portuguese?), try to use simple words and phrases that you are comfortable with, and do your best to give a clear explanation, but I do understand that it can be difficult.